The present invention relates to a tire intended to be fitted on a vehicle and more particularly intended to be fitted on a two-wheeled vehicle such as a motorcycle.
Although not limited to such an application, the invention will be more particularly described with reference to such a motorcycle, or motorbike, tire.
Tire reinforcement armatures or tire reinforcements, in particular for motorcycle tires, are currently—and most frequently—formed by stacking one or more plies conventionally designated “carcass plies”, “crown plies”, etc. This way of designating reinforcement armatures stems from the manufacturing process, which consists of making a series of semi-finished products in the form of plies, provided with, often longitudinal, cord reinforcements, which are then assembled or stacked so as to build a tire blank. The plies are made flat, with large dimensions, and are then cut as a function of the dimensions of a given product. The plies are also assembled substantially flat in a first phase. The blank thus produced is then shaped into the toroidal profile typical of tires. The semi-finished products known as “finishing products” are then applied to the blank, in order to obtain a product which is ready for vulcanization.
Such a “conventional” type of process involves, in particular for the tire blank manufacturing phase, the use of an anchoring element (generally a bead wire), used to effect anchoring or retention of the carcass reinforcement in the tire bead zone. Thus, for this type of process, a turn-up is formed from a portion of all the plies constituting the carcass reinforcement (or of only some) around a bead wire disposed in the bead of the tire. The carcass reinforcement is thereby anchored in the bead.
The widespread use in the industry of this type of conventional process, despite numerous variants in the manner of producing the plies and the assemblies, has led the person skilled in the art to use a vocabulary derived from the process; hence the terminology in general use, comprising in particular the terms “plies”, “carcass”, “bead wire”, “shaping” to designate transition from a flat profile to a toroidal profile, etc.
Tires now exist which do not strictly speaking comprise “plies” or “bead wires”which fit the above definitions. For example, document EP 0 582 196 describes tires manufactured without the aid of semi-finished products in the form of plies. For example, the reinforcement elements of the various reinforcement structures are applied directly onto the adjacent layers of rubber mixes, the whole being applied in successive layers onto a toroidal core whose shape allows the direct obtainment of a profile resembling the final profile of the tire in the process of being manufactured. Thus, in this case there are no longer “semi-finished products”, or “plies” or “bead wires”. The basic products such as the rubber mixes and the reinforcement elements in the form of cords or filaments are applied directly to the core. Since this core is toroidal in shape, the blank no longer has to be shaped in order to be transformed from a flat profile to a profile in the form of a torus.
Furthermore, the tires described in this document do not have the “traditional”carcass ply turn-up about a bead wire. This type of anchoring is replaced by an arrangement in which circumferential cords are disposed adjacent to said sidewall reinforcing structure, the whole being immersed in an anchoring or bonding rubber mix.
Assembly processes using a toroidal core also exist which use semi-finished products specially adapted for rapid, effective and simple laying on a central core. Finally, it is also possible to use a composite comprising certain semi-finished products to achieve certain architectural aspects (such as plies, bead wires, etc), while others are achieved by the direct application of mixes and/or reinforcement elements.
In the present document, so as to take account of recent technological developments both in the field of manufacture and in the design of products, the conventional terms such as “plies”, “bead wires” etc are advantageously replaced by neutral terms or terms which are independent of the type of process used. Thus, the term “reinforcement of carcass type” or “sidewall reinforcement” may be used to denote the reinforcement elements of a carcass ply in the conventional process and the corresponding reinforcement elements, generally applied at the level of the sidewalls, of a tire produced according to a process not using semi-finished products. The term “anchoring zone”, for its part, may denote both the “traditional” carcass ply turn-up about a bead wire of a conventional process and the assembly formed by the circumferential reinforcement elements, the rubber mix and the adjacent sidewall reinforcement portions of a bottom zone produced using a process involving application on a toroidal core.
As in the case of all other tires, we are witnessing the radialization of motorcycle tires, the architecture of such tires comprising a carcass reinforcement formed of one or two plies of reinforcement elements forming with the circumferential direction an angle which may be between 65° and 90°, said carcass reinforcement being radially surmounted by a crown reinforcement formed at least of generally textile reinforcement elements. Non-radial tires do still exist, however, and the invention also relates to them. The invention also relates to partially radial tires, that is to say whose reinforcement elements of the carcass reinforcement are radial over at least part of said carcass reinforcement, for example in the part corresponding to the crown of the tire.
Numerous crown reinforcement architectures have been proposed, depending on whether the tire is intended to be fitted at the front of the motorcycle or at the rear. A first structure consists, for said crown reinforcement, in using solely circumferential cables, and said structure is more particularly used for the rear position. A second structure, directly inspired by structures commonly used in passenger-vehicle tires, has been used to improve wear resistance, and consists in using at least two crown plies of reinforcement elements which are parallel to each other within each ply but crossed from one ply to the next, forming acute angles with the circumferential direction, such tires being more particularly suitable for the front of motorcycles. Said two crown plies may be surmounted radially by at least one ply of circumferential elements, generally obtained by helical winding of a strip of at least one reinforcement element coated with rubber. Patent no. FR 2 561 588 thus describes such a crown reinforcement, having at least one ply whose reinforcement elements form with the circumferential direction an angle which may vary between 0° and 8°, the modulus of elasticity of such elements amounting to at least 6000 N/mm2, and, disposed between the carcass reinforcement and the ply of circumferential elements, a shock-absorption layer formed mainly of two plies of elements crossed from one ply to the next forming between them angles of between 60° and 90°, said crossed plies being formed of textile reinforcement elements having a modulus of elasticity of at least 6000 N/mm2.
Document EP 0 456 933, with a view to providing a motorcycle tire with excellent stability at high speed as well as excellent contact with the ground, teaches for example that a crown reinforcement should be made with at least two plies: a first ply, radially the closest to the carcass reinforcement, being composed of cables oriented at an angle of between 40° and 90° relative to the circumferential direction and the second ply, radially the closest to the tread, being composed of cables wound helically in the circumferential direction.
U.S. Pat. No. 5,301,730, with a view to enhancing the drive ability of a tire designed for the rear of a motorcycle, proposes a crown reinforcement composed, proceeding from the radial carcass reinforcement to the tread, of at least one ply of substantially circumferential elements and two plies of elements crossed from one ply to the next and forming with the circumferential direction an angle which may be between 35° and 55°, elements of aromatic polyamide being suitable for use as the ply of elements parallel to the circumferential direction and aliphatic polyamide being suitable for the plies of crossed elements.
The production of motorcycle tires results in large curvature values if such tires are to be used on a camber. The invention is directed in particular at improving the properties of tire grip and drive ability for use on severe camber.
Studies carried out in this context have shown in particular that the presence of layers of reinforcement elements forming an angle with the longitudinal direction results in local circumferential and shearing rigidities which diminish near the edges of said layers, the tension at the ends of the reinforcement elements being zero. A local tension of zero for the reinforcement elements results in reduced effectiveness of said reinforcement elements in this zone. The rigidities of the edges of the layers are particularly important when the tire is used on the most severe cambers, when cornering, the part of the tire corresponding to said zones then facing the ground.
The invention aims to improve further the quality of motorcycle tires, without for all that impairing the other properties required to satisfy the users. More specifically, the invention is directed at improving tires comprising at least one layer of reinforcement elements forming an angle with the longitudinal direction.